If a business user shares a folder with a personal user
No. As the business shared folder will not show up in the personal OneDrive “shared” section, the personal user can’t click “add to my OneDrive” to sync it.

If a personal user shares a folder with a business user
No. As the personal shared folder will not show up in the OneDrive for Business “shared with me” section, the business user can’t click sync to sync it.

If a business user shares a folder with a business user
Yes. As the business shared folder will show up in the OneDrive for Business “shared with me” section, the business user can select the folder to sync it.

If a personal user shares a folder with a personal user
Yes. As the personal shared folder will show up in the personal OneDrive “shared” section, the personal user can click “add to my OneDrive” to add the folder to “File” section to sync it.